Human Eye Structure and Function-Function of the Human Eye
Human eye structure and function. Eye is a one of the sensory organ and the structure of eye is complex. The eye is situated in orbital cavity. It is an organs of the visual system. It helps in the vision, ability to receive and process visual detail and enabling several photo functions dependent of visions. Eye is a sensitive part of human body.
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| HUMAN EYE |
Key points on eyes:
Total power of eye is 60 D (cornea 45 D + lens 15D)
Diameter of eye is 2.5 cm & IOP is 10-21 mm of Hg
Weight & volume is 7 gm & 6.5 ml/cc
Normal visual acuity is 6/6
Test for visual acuity or far vision is Snellen's chart
Test for near vision is Jegher's chart
Test for colour vision is Ischihara chart
WHO blindness is <3/6
Far point of eye is Infinite
Near point of eye is 25 cm
Cornea is also called Window of eye
Conjunctiva: Protective layer which covers Sclera and inner eyelids
The structure of eye can be divided into external and internal.
External structure of eye:
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| External structure of eye / Anatomy |
Eye is externally composed of the white part Sclera, coloured part Iris, Pupil, upper & lower Eyelids and the junction between upper and lower eyelids Canthus.
Internal structure of eye:
Internally the eye is made of majorly 3 layers i.e. Outer / Fibrous layer, Middle layer, Inner layer.
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| Internal Structure of eye |
Outer Layer:
This a fibrous avascular layer of eye consisting Sclera posterior 5/6 parts and Cornea anterior 1/6 parts. Sclera and cornea get nutrition through aqueous humour which is present in anterior chamber.
The junction between Sclera and cornea is called Limbus, which is called Window of eye.
The donation of eye means Cornea donation which is called Keratoplasty. Cornea is made of 5 layers:
1. Epithelium Layer: It is surface layer of cell, Which provide barrier function & a smooth surface for the tear film.
2. Bowmans Layer: Tough layer of basement membrane just under the epithelium layer. This layer is tough, & prevent the cornea from swelling forward, which means when the cornea swells...it must do so backwards into the anterior chamber.
3. Stroma: It is composed of arranged collagen fibers and supporting keratocytes, The stroma provides the majority of the cornea. Injuries at this level can scars.
4. Descemet’s Layer: This an inner layer of basement membrane. Descemet’s Layer is important for the health of endothelial cells. One of the leading needs for cornea transplant is from a dystrophy of Descemet’s layer which is called Fuch’s dystrophy.
5. Endothelium layer: This is a crucial layer of cornea. It works as a barrier and a pump, keeps the cornea from getting too wet.
Middle Layer or Vascular layer:
The middle layer is Made of 3 layers i.e. Choroid, Ciliary muscle and Iris called Uvea. Inflammation of these layers called Uveitis.
1. Choroid: This is highly vascular and pigmented layer and occupied maximum part of middle layer of eye.
2. Ciliary muscle: Formation of aqueous humour and helps in accomodation and maintain intraocular pressure (IOP). Aqueous humour drainage through Canal of Schlem.
3. Iris: It provides colour of eye and control the pupil (contract & dilate). The normal size of pupil is 3-5 mm. Pupil is controlled by a muscle called Sphinter Pupillae.
Inner layer or Retina:
The inner layer of eye is Retina whereas image forming complete. There is two Spots in Retina.
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| Healthy Retina |
Yellow spot or maculla called Fovea centralis. Here is present Rod cells and Cone cells, and Blind spots here is present of nerve cells.
Rod cells:
- 120 millions in number
- Helps in dim light vision
- Pigmented due to Rhodopsin
- Deficiency results in Night blindness
Cone cells:
- 6 millions in number
- Helps in day light vision or colour vision
- Pigmented due to Idopsin
- Deficiency results in Colour blindness
Glands of eye:
1. Gland of zeis
2. Lacrimal gland
3. Meinomian gland
4. Others: gland of moll
Largest glands of eye is Lacrimal gland which secrets tears having antibacterial enzyme called Lysozyme. Lacrimal gland pours it secretion in lacrimal sac.The opening of lacrimal sac is called Puncta.
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| lacrimal gland |
Eye muscle / extra occular muscles are 6 in number.
1. Superior rectus
2. Inferior rectus
3. Lateral rectus
4. Medial rectus
5. Superior oblique
6. Inferior oblique
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| Eye muscles |
Cranial nerve which helps in eye movements is mainly Occulomotor 3rd cranial nerve. 6 th cranial nerve Abducent and 4th cranial nerve Trochlea also helps in eye movements.
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